1. Field of the invention
The invention concerns a timing recovery device for receiver installations using adaptive equalization and oversampling associated with differentially coherent demodulation. It applies in particular to dispersive multipath digital data transmission causing intersymbol interference in the received data, for example digital microwave transmission on a long line of sight path or a tropospheric path. It is applicable to transmission systems which use differentially demodulatable linear modulation such as QPSK or MPSK and advantageously receiver installations which can operate in diversity mode.
2. Description of the prior art
The presence of intersymbol interference tends to degrade significantly the performance of the receiver installations used in digital microwave transmission. A known way to remedy this drawback in the case of coherent modulation is to use adaptive equalization by means of an adaptive filter whose function is to minimize intersymbol interference. In the systems that are the most widespread because they are the simplest the filter optimization criterion is the mean square error (which is to be minimized) and the filter coefficients are updated by the stochastic gradient algorithm.
A known way to improve further the performance of receiver installations using coherent demodulation and adaptive equalization is to use the diversity technique by including in each receive channel an adaptive equalization filter processing the received signal and to connect the output of the demodulator to a so-called decision feedback adaptive filter which processes the symbols decided on. In the case of coherent demodulation the filter coefficient adaptation algorithm is well known and many receiver installations have been implemented to date. One example of a receiver installation using diversity and adaptive equalization is described in U.S. Pat. No. 3,879,664.
The drawback of coherent demodulation is that the phase of the received signal must be estimated. This is particularly difficult because the paths in question cause fast variations in phase which can desynchronize the receiver.
One known solution to the problem of avoiding this estimation of the phase of the received signal is to use differentially coherent demodulation, more usually referred to as "differential demodulation". In this technique the previous sample is used as a phase reference for the signal sample being processed. A receiver installation using adaptive equalization in association with differential demodulation is described in French patent application FR-88 17507.
For receiver installations using adaptive equalization in association with differential demodulation the receiver clock or timebase must be synchronized to the transmitter timebase which controls the timing of data symbol transmission. Timing recovery devices are known which operate on the complex envelope of the received signal prior to equalization.
These known devices offer poor performance in the case of a dispersive transmission path, however.
Also, diversity mode operation of the receiver requires a combination of estimates and this makes the receiver complex. Furthermore, because sampling time control is independent of equalization, the sampling time set by the timing recovery device means that the equalizer cannot operate under optimum conditions. In particular, skipping of successive cycles (occurring after a particular time) tends to desynchronize the equalizer which makes it impossible to guarantee correct operation of the receiver in the long term.
This type of timing recovery device is suitable for equalizers with a low correction capacity for digital transmission via low dispersion paths but not equalizers for digital transmission via highly dispersive paths such as the tropospheric path.
Other known timing recovery devices use the coefficients of the transversal filters of the equalizer to correct the phase of the recovered timing on the basis of a filter group propagation time control criterion as described in the document IEEE Transaction of Communication, vol--COM-24, No 8, August 1976.
Timing recovery devices of this kind have various drawbacks:
For an equalizer operating in diversity mode and governed by a single error which is generated after combination, the group propagation time of each filter no longer conveys data and not all combinations of data are able to supply a usable recovered timing phase error estimate signal.
The acquisition range and bandwidth performance of the timing control loop are limited by the loop bandwidth of the equalizer which is just a few hertz. A timing recovery device of this kind requires the use of highly stable clocks if the performance of the equalizer is not to be degraded because the coeffients of the filters are varied to adapt to random variations in the phase of the clock.
One solution to making the adaptive equalizer relatively insensitive to the signal sampling time is to oversample the received signal prior to equalization at a frequency which is a multiple of the symbol frequency. Two times oversampling is most commonly used. In such a scheme it might be thought that timing recovery is no longer necessary because it is carried out by the equalizer.
However, this would require the use of an equalizer with an unlimited number of coefficients. This solution is not feasible because the number of filter coefficients must be finite.
An object of the invention is to remedy this drawback.